Silencer



Aug.10,1'943. RBBOURNE 2,326,612

S ILENCER Filed Nov. 25, 1940 CYLZEJ Haicom Patented Aug. 10, 1943 SILENCER Roland B. Bourne, West Hartford, Conn., assignor to The Maxim Silencer Company, Hartford a corporation of Connecticut Conn.,

Application November 25, 1940, Serial No. 367,054

21 Claims.

In my prior Patent2,043,731, June 9, 1936, I have described an improvement in sound attenuating devices which comprises the use of closed side branches or resonator chambers, the coupling of which to the main sound conducting channel was solely through a body of sound absorbing material. In the best forms of the device there shown the chambers were made in fairly short lengths. In these cases the most desirable structural form consisted of a circular main conduit around which annular sidebranches were positioned, the conduit being perforated .throughout substantially the entire length of each sidebranch and having arranged externally of the perforations a layer of sound absorbing material generally held in place by a second layer of perforated metal. It was found that if the density of the sound absorbing material was properly chosen an attenuation substantially uniform throughout a very wide frequency range could be obtained. While higher attenuation at low frequencies could be attained by decreasing the amount of sound absorbing ymaterial a limited degree below this normal amount, the attenuation at very high frequencies (higher than is indicated in Fig. l2 of that patent) would suffer; and while the attenuation at the higher freyquencies could be improved by increasing the amount of sound absorbing material, the attenuation at the lower frequencies was adversely affected thereby.

In my application, Serial No. 90,662, flied July 5, 1936, I described an improvement on this earlier construction consisting of the use of one or more sound absorbing partitions within the resonating chamber to prevent the formation of longitudinal standing waves therein. These standing waves, occurring at .frequencies for which the sidebranch Was an integral number of half Wave lengths long, caused a general loss of attenuation within certain frequency ranges and also produced localized dips in the frequency-attenuation curve. This type of construction increased substantially the attenuation possible for a given size.

The present invention is an improvement upon the structures of these earlier disclosures in that a much greater and more uniform attenuation can be secured with a device of given size than would be possible with even the best embodiments of the earlier types. The complete theory upon which the device operates is too dicult mathematically to permit an exact quantitative discussion, but it is believed that a good picture of the operation of the improved device, together with certain limiting conditions, can be given with ample accuracy to permit'the construction of these devices in varying requirements.

The present improvement in one aspect desizes and for varying pends upon a novel manner of using a greater density of sound absorbing material in av silencer of given size (as for example by increasing the density of its packing) without decreasing the efciency of the silencer at low frequencies. In this particular the invention depends upon the discovery that both free and resistive coupling intoa single sidebranch can be employed simultaneously with substantial advantage, provided the proper balance between them is maintained. While this balance is not at all critical, it has cert-ain limits which must be preserved if the full advantages of the invention are to be obtained. Briefly, a silencer constructed in accordance with the present invention utilizes a sidebranch coupled to a main sound conducting channel through two conductivities in parallel. One of these conductivities consists of a localized opening or openings, and the other consists of a distributed body of sound absorbing material. Where the sidebranch is positioned in annular form around the main channel, as in the preferred constructions, the distributed mass of sound absorbing material should preferably extend substantially throughout the common boundary of the sidebranch and the main channel. The localized unrestricted openings may apparently be placed at any point along the length of the sidebranch, but in general improved results are obtained by placing these openings approximately in the middle of the sidebranch. In order to balance the two types of conductivities it is desirable to have the density of the sound absorbing material rather higher than was formerly possible; a preferred construction in a one-inch pipe size silencer, for example, embodying rock wool packed between two perforated metal cylinders having preferably more than 10% of their surface perforated, the rock Wool being preferably packed as tight as is conveniently done by hand. A fibrous material like rock wool or felt will be found generally advantageous in that it permits somewhat denser packing than do such materials as vermiculite and various clay products. The localized, unrestricted openings are preferably larger than the minimum value discussed below and are preferably of such a size that their conductivity, if measured by the eifect on the resonating characteristics of the chamber, will be on the order of one to ten times as great as the conductivity into this resonance chamber provided by the sound absorbing material.

The upper limit of the unrestricted openings may be determined in accordance with standard practice of proportioning the coupling openings into volumetric sidebranches, it being Well understood that beyond a certain point no advantage, and even a detriment, is obtained by increasing the conductivity. In the present case an additional disadvantage of increasing the area of the unrestricted openings too greatly is found in the fact that these openings are providedl at the expense of sound absorbing material, and

the chamber approximately midway of the extent of the chamber along the channel, this partition being preferably positioned to permit direct access of sound Waves between the main channel and the portions of the chamber to either side of the partition. l

The invention will now be described in connection with the accompanying drawing, in which: l

Fig. 1 is a diagrammatic longitudinal section through a single section vsilencer constructed in accordance with the disclosure of my prior Patent 2,043,731 but with specific dimensions thereon to make available a direct comparison with the present device;

Fig. 2 is a similar view showing a modification thereof in accordance with the teachings of my prior application Serial No. 90,662;

Fig. 3 is a similar view showing a silencer constructed in accordance with the present invention;

Fig. 4 is a section on line 4 4 of Fig. 3;

. Fig. 5 is a graph showing a comparison between the operating characteristics of the device shown in Figs. 1, 2, and 3; and

Fig. 6 is a diagrammatic longitudinal sectionl through a four-section silencer embodying the present invention in a slightly different physical form.

It is believed that an understanding of the invention and its functioning 'can best be reached by considering its operation in comparison with prior types. The device of Fig. l is a single section silencer substantially that shown in my Patent 2,043,7'31 but with illustrative dimensions added to the drawing. It will'be understood that these dimensions, which are applicable Aalso to the specic devices of Figs. 2 and 3, are not intended to restrict the proportions of the device but only to describe as completely-as possible one form of the invention and to makev a comparison possible between this form and comparable examples of the prior art. The one-quarter inch annular space between the two layers of perforated metal I and II is lled with rock wool I2. 'Ihe space within the inner layer I 0 of perforated metal forms the main sound conducting channel and connects with pipes I3 and I4, one of which may serve as an inlet and the other as an outlet. The whole is surrounded by a casing I5, providing a space I6 serving as a sidebranch or resonator. A graph showing the frequency versus attenuation characteristics of this silencer, with the rock wool packed to a density of approximately onesixth of an ounce per cubic inch, is shown in Fig. at A.

It will be observed that this device shows a dip I1 in the frequencyattenuation curve at about 2,000 cycles; and both for the purpose of preventing the occurrence of this dip and to increase the general attenuation the device of Fig. 2 has added to it a central pervious partition I8 in acy cordance with the disclosure of my application,

Serial No. 90,662. A curve illustrating the effect of this partition, which may be made of felt, or of rock wool or other sound absorbing material held in place by permeable walls, is shown in Fig. 5a B.

A structure embodying the present invention has been shown in Fig. 3. i It will be observed that this diiers from the previously described device in that the layer of sound absorbing material 20 (here packed to a density of one-third of an ounce per cubic inch) does not extend completely throughout the length of the sidebranch, but is interrupted by an annular gap or slot 2|. To prevent blocking the outer end of this slot, the outer pervious partition 22 is preferably, butnot necessarily, slightly shifted longitudinally as shown in the drawing, and also preferablyterminates short of the outer layer 23 of perforated metal in order to provide an annular gap 24 insuring adequate coupling directly from the slot 2|.to the part of the resonator chamber beyond the partition. A frequency-attenuation graph of this device has been shown on Fig. 5 at C. It will be observed that the attenuation has been raised materially throughout the sound spectrum by the combination of free and resistive coupling, the gap 2| of course securing the freecoupling and 'the body 20 of sound absorbing material providing the resistive coupling. The improvement made by the construction .of Fig. 3 is denitely brought out in the graphs.

It is necessary that the amount of free coupling be sufficient-to maintain high attenuation in the lower frequencies in spite of the density vof the sound absorbing material. There are i various methods by which this conductivity may be gauged and it may be said that its exact amount is not critical, provided that the gap is large enough to accomplish its intended purpose and is not so large as to destroy the characteristics of the device. While for a silencer of any lgiven-size it would be easy to give a minimum or even an optimum figure for the width of slot, it is not'possible to do this with generality for al1 sizes as the necessary width of slot will vary in accordance with the size and Shape of the silencer being built. For example, a very satisfactory size of slot in a silencer of the size shown in Fig. 3 is three-eighths of an inch, but this would be entirely inadequate for a silencer of twelve-inch pipe size.

Another way of stating the same thing is to consider the relationship between the conductivity offered by the slot and that oiered by the sound absorbing material. It has been found by experiment that the conductivity oiered by the slot should be at least equal to, and preferably should be Aat least five times as great as the conductivity of the sound absorbing material. The acoustic conductivity of anopening into a chamber can be dened for the purposes of this discussion by the formula w2V C0: -C-'z where Co is the conductivity,

w is 2- times the resonance frequency of the chamber, V is the volume of the chamber, and C is the velocity of sound in the medium. There are various more or less rough tests which will give a proper range of relative s ize for the coupling zones through the sound absorbing material and through the' unrestricted opening. It has been stated that.the conductivity, as defined above, should be at least as great through the opening as through the sound absorbing material. The best balance on devices of the illustrative size shown has been found to be a ratio of about six to one for the free and restrictive conductivities. Another way of looking at the system is to consider that the opening should be large enough to develop in the chamber at its resonance frequency substantially its full attenuating eifect on sou'nd waves in the main channel. If the opening into a side branch is progressively increased from zero, a progressive increase in the attenuation at resonance will at iirst result; the attenuation being, however, less and less ailected by succeeding increases in the conductivity of the coupling. There is, so t speak. a knee `in the curve of attenuation vs. coupling; above which the increase in attenuation is relatively slight in proportion to the increase in conductivity. In the present case an increase in the width oi' the slot necessitates the reduction in the area of the coupling devoted to sound absorbing material and it is therefore undesirable to have the opening larger than is necessary to develop by itself substantial resonance effects.` A further disadvantage in increasing the area of free opening beyond what is necessary to secure adequate attenuation is that 4 by increasing the conductivity into the sidebranch the resonance frequency is raised, which is undesirable. As far as I am aware no text on acoustics has presented any formula by which the requisite amount of coupling for obtaining a suiiicient grip on the sidebranch can be calculated, but the matter is easily settled by trial for any given type and size of main channel and sidebranch. A rough guide is that the area of the opening should be at least one-quarter of,

and preferably on the order of, the cross-sectional area of the main channel. In a silencer of one-inch pipe size as shown in the drawing the one-quarter rule gives a slot width of one- -sixteenth inch, which experiment shows is just about as low as can be used without discarding entirely the advantages ofthe invention. It is preferable, however, to use in this case an opening on the order of the area of the main channel, or in the illustrative case aboutthree-eighths inch, in order to secure the full advantages of the invention. An alternative dennition is that the area of the unrestricted opening or openings should be more than a ftieth of the area of sound absorbing material exposed to the main channel. 1

Of course, for the purposes of obtaining special results. such as the accentuation of various frequencies. or in order to accommodate various commercial requirements, the conductivities can be varied as desired but the case discussed is the general one Where a generally uniform attenuation is desired throughout the sound spectrum. This will enable the silencer to be applied indiscriminately to sound sources of varying frequency distribution. A silencer constructed in accordance with the present invention possesses this uniform attenuation to a high degree over a major portion of the sound spectrum. The increase in attenutaion for a given unit, as compared with a construction such as shown in Fig. 1, makes it possible either to build a more compact device or to produce a. higher degree of attenuation in a device of given size as may be desired.

For the purpose of permitting variations to be made to suit particular problems of design, and to explain further the manner in which the device operates, the eii'ect of changing the various elements will be discussed briefly. vThe width of the slot 2|, or the area of an equivalent free conductivity, can be varied through rather wide limits, the optimum, for the size of silencer shown in the drawing, being about three-eighths of an inch. For a slot width of one-sixteenth of an inch the attenuation suifers materially except in the frequencies above 2000 cycles, where the eifect of even a thin layer of sound absorbing material becomes more and more pronounced.

For a wider slot such as one-half inch the at.

tenuation is increased in a narrow range of intermediate frequencies due to resonant action, but suii'ers both in the high and low frequency range.

It is easy to see the result of varying the narrowing or widening of the slot to the limit. Narrowing the slot to zero is equivalent to producing the device of Fig. 2, and the approach to this in the main will be progressive as the slot becomes narrower. Widening the slot changes the sidebranch chamber to a mere enlargement in the main channel, which characteristically produces attenuation for limited frequency bands with greatly reduced attenuationelsewhere. The widening of the slot of course also progressively destroys the attenuating action of the sound absorbing material 20.

'I'he thickness of the layer of sound absorbing material is also variable, the thickness being a compromise between the increase of attenuation directly due to the added sound absorbing material and the decrease of attenuation due to the decrease in volume of the resonator chamber. For the material and the size of silencer described, a quarter of an inch thickness gives good results, and this will serve as a guide where other materials or proportions are being employed.. l

I he density with which the sound absorbing material is packed has a marked effect upon the performance of the device. In the size shown for illustration (it being remembered that size controls the resonating frequency of the sidebranch) a gradual decrease in the density of packing will cause a general decrease in eilicien'cy which is particularly noticeable in the critical range between 250 and 1500 cycles where it is particularly diiiicult to get uniform and at the same time high attenuation. A change of sound absorbing material will of course introduce variation of performanceA dependingupon the particular material chosen, but which can easily be determined by tests made along the lines indicated.

'I'he partition 22 is a further controllable element. In general .it may be said that the best results are obtained by having the partition only thick enough to prevent standing waves being set up, with their consequent reinforcement and by-passing of those frequencies for Which the resonating chamber is an integral number of half-Wave lengths long. An increase in partition thickness above this point is of value only in the case of adding attenuation (generally unnecessary) in the higher frequency range, the resulting decrease in the volume of the resonator chamber causing poorer attenuation at the lower frequencies. The thickness of the partition can be reduced, or the partition omitted entirely, if the loss in attenuation at the series-resonant frequency is not regarded as serious. Indeed, even with this localized reduction in attenuation resultln'g from the omission of the partition the `aaaacie type shown in Fig. 1. Particularly for silencers 'Y having their chambers long with respect to their diameters, and in any case Where the vsecond or higher harmonics of the longitudinal resonance frequency is of importance, it maybe desirable to add one or more additional partitions-located as near as may be to anti-nodal points for the longitudinal standing waves within the resonator.

The size of the chamber of course directly aiects the attenuation at the frequencies to which the chamber is resonant. In particular, by increasing the diameter vof the chamber its resonant frequency is lowered and the attenuation offered by the device is improved at the lower frequencies." The attenuation can be increased at will by adding more sections in series along the main channel, the eiect being practically additive if care is taken to prevent mechanical transmission of sound energy from one section to the next.

A further manner in which the invention may be varied, enabling the partitions 22 to be dispensed with in man3r cases, is shown in Fig. 6. This iigure shows a shell 30 having intermediate annular partitions 3|. To each of these partitions is secured, so as to surround its central hole, a retainer comprising two concentric sleeves 32 and 33 and containing sound absorbing material 34 such as rock wool as described above. 'Iihe end of each retainer is spaced from the adjacent partition (or, in case of the end one, from the header 35) so as toleave an annular slot 36. The functioning of this device is as described above with the exception that the.slot 36 is at the end rather than at the center of the chamber and that the partitions have been omitted. Little diierence in operation is produced by this change. In order to avoid the use of sound absorbing partitions within the annular resonance chambers 31 the chambers are made of unequal lengths by properly spacing the partitions 3l. While this does not eliminate the undesirable eiects of series or longitudinal resonance as completely as could be done by the partitions, it in many cases will do so with sufficient thoroughness for practical purposes by causing the dips in the attenuation curves of the several chambers to be shifted so that they will not overlie one another.

It will be understood that the sound absorbing material may be chosen as desired for the particular use to which the silencer is to be put and that with this limitation any of the standard materials, such as felt, vermiculite, metal wool, or the like, may be substituted for rockwool.

Iclaim:

1. A silencer having a main vsound conducting channel, a chamber having a substantial extent along the main channel, a partition of sound absorbing material positioned across the chamber substantially midway of its extent along the main channel, and alcoupling between the main channel and said chamber composed at least in part of one or more substantially unrestricted openmgs.

2. A silencer having a main sound conducting channel, a chamber having a substantial extent along the main channel, a partition of soundv absorbing material positioned across the. chamber substantially midway of its extent along the main channel, and a coupling betweenthe main channel and said chamber composed in part of one or more substantially unrestricted openings andv in part of sound absorbing material.

3. A silencer having a main sound conducting enamel, a chamber-.having a substantial extent along the main channel, a partition of sound absorbing material positioned across the chamber substantially midway of its extent along the main channel, and a coupling between the main channel and said chamber composed in part o! one or more substantially unrestricted openings and in part of sound absorbing material, said openings l being localized approximately midway of the extent ofthe chamber along the channel and having direct' connection with the chamber on each side of the partition.

4. A silencerhaving a main sound conducting channel, a chamber having ya substantial extent along the main channel, a partition of soun'd absorbing materialfpositioned across the lchamber substantially midway of its extent along the main channel, and a coupling between the main channel and said chamber composed in part of one or more substantially unrestricted openings and in part of sound absorbing material, the conductivity of the opening being at least as great as the conductivity of the sound absorbing material.

5. A silencer having a main sound conducting channel, a chamber having a substantial extent along the main channel, a partition of sound absorbing material positioned across the chamber substantially midway of its extent along the main channeL'and a coupling between the main channel and said chamber composed in part of one or more substantially unrestricted openings and in .part of sound absorbing material, the area of the openings being greater than a quarter of the area of the main channel and the sound absorbing material having sufcient density to cause the conductivity of the lsound absorbing material to be below that of the openings.

6. A silencer having a main sound conducting channel, a chamber having'a substantial extent along the main channel, a partition of sound ab'- sorbing material positioned across the chamber substantially midway of its extent along the main channel, and a coupling between the main channel andsaid chamber-composed in part of one or more substantially unrestricted openings and in part of sound absorbing material, the conductivity of the openings being on the order of live times that of the sound absorbing material.

7. A silencer having a main sound conducting channel, a chamber having a substantial extent along the main channel, a partition of sound absorbing material positioned across the chamber substantially midway of its extent along the main channel, and a coupling between the main channel and said chamber composed in part of one or more substantially unrestricted openings and in part of sound absorbing material, said openings being localized approximately midway of the extent of the chamber along the channel, the conductivity of the opening being at least as great as the conductivity of the sound absorbing material.

8. A silencer having a main sound conducting channel, a chamber having a substantial extent along the main channel, a partition oi sound absorbing material positioned acrss the chamber substantially midway of its extent along the main channel, and a coupling between the main channel and said chamber composed in part of one or more substantially unrestricted openings and in part of soundabsorbing material, said openings being localized approximately midway of the extent ofthe chamber along the channel, the area channel, and a coupling between the main channel and said chamber composed in part of one or more substantially unrestricted openings and in part of sound absorbing material, said openings being localized approximately midway ofthe ex- .tent of the cha'mber along the channel, the conductivity of the openings being on the order oi ve times that oi' the sound absorbing material.

10. A silencer comprising a main sound conducting channel, a chamber having a substantial length along the main channel, the boundary between the channel and the chamber being formed of pervious sound absorbing material having at least one discrete opening therethrough intermediate the length of the chamber, and at least one pervious transverse partition located within the chamber intermediate its length.

ll. A silencer comprising a main sound and gas conducting channel lined with pervious sound absorbing material, a pervious outer container for said sound absorbing material, an outer chamber acoustically coupled to said main sound and gas conducting channel partly through said sound absorbing material and partly through an unrestricted opening therethrough, the area of said unrestricted opening being at least onequarter of the cross sectional area oi said main sound and gas conducting channel.

12. A silencer comprising a main sound and gas conducting channel lined with .pervious sound absorbing material, a pervious outer container for said sound absorbing material, an outer chamber acoustically coupled to saidy main sound and gas conducting channelv y through said sound yabsorbing material and partly through an unrestricted opening therethrough, the area ot said unrestricted opening being on the order of the cross sectional area oi' said main sound and gas conducting channel.

13. A silencer comprising a main sound conducting channel alined with pervious sound absorbing material. apervious outer container for said sound absorbing material, and an outer annular chamber acoustically coupled to said main channel partly through the sound absorbing material and partly through an annular unrestricted opening positioned at a point intermediate the length of said outer chamber. s

14. A silencer comprising a main sound and gas conducting channel and a closedacoustic sidebranch acoustically coupled thereto partly through a layer oi sound absorbing material and partly through an unrestricted opening, said sidebranch having a body of sound absorbing material positioned therein at a point in adjacency to said unrestricted opening.

15. A silencer comprising a main sound conducting channel acoustically coupled to a closed chamber partly through a body of sound absorbing material and partly through an unrestricted opening in parallel therewith, said opening having an area at least one-quarter o f that oi the main channel, and a pad of sound absorbing material disposed in said chamber in adjacency to said unrestricted opening.

16. An acoustic sidebranch'comprising a closed chamber, a pervious body oi sound absorbing material forming a boundary of said chamber and providing a resistive coupling thereinto, said body being pierced by one or more openings forming a second coupling into the chamber, and a pervious partition located within the chamber adjacent to said second coupling.

17. An acoustic sidebranch comprising a closed chamber,` a pervious body of sound absorbing material forming a boundary of said chamber and providing a resistive coupling thereinto, said body being pierced by one or more openings forming a second coupling into said chamber, and a pervious incomplete partition located within said chamber adjacent to said second cou- Y pling to divide the chamber while permitting direct access to each division through said second coupling.

18. A silencer comprising a main sound conducting channel, `a resonator, and a resistive and an unrestricted coupling passage in parallel. each acoustically coupling the channel to the resonator, the acoustic conductivity oi' the unrestricted coupling passage being atleast as great as the acoustic conductivity of the resistive coupling passage.

19. A silencer comprising a main sound conducting channel, a resonator, and a resistive and an unrestricted coupling passage in parallel, each acoustically coupling the channel to the resonator, the acoustic conductivity of the unrestricted coupling passage being on the order oi' ilve times that of the acoustic conductivity oi the resistive coupling passage.

20. In an acoustic system, a main sound conducting channel, a. resonating chamber coupled thereto directly through an open aperture, and

a pervious body of sound absorbing material located within the chamber adjacent said aperture intermediate the walls of the chamber and at a point of high acoustic velocity for a standing wave system therein, said body extending subr stantially but not completely across the chamber to leave a gap, permitting the said direct coupling between the main channel and said chamber.

21. In an acoustic system, a main sound conducting channel, a resonating chamber having a substantial extent longitudinally oi the channel and coupled directly thereto at least through an open aperture located substantially midway of the length of the channel, and a pervious body of sound absorbing material Within the chamber adjacent said aperture intermediate the walls of said chamber and at a point of high acoustic velocity for a standing wave system therein, said body extending substantially but not completely across the chamber to leave a gap permitting the said direct coupling between the main channel and said chamber.

ROLAND B. BOURNE.

CERTIFICAT oF coRREcTroNv.'

` August 1o, 1915.

mamme'. Boum.

It is hereby certified vthat error apjaears vin the printed specification of theabove numbered' p'atef; requiring correction-ns fo11ow-e: -Pege 5, first I umn, line 52, claim lluforhalined" read #-lined--g and lshalsthe'sslidA Letore Patent lshould be read with A.this correction therein that the san1e'may cor14 from to the record of the case in the Patent Office. A

Signed and sealed this ilth day of `September, A. D. 1915..A

. Henry van Aradale., y

(seal) 'Acting Commissioner ofi Patents.

cERTIFIcAT oF CORRECTIQNQ August 1Q, 1911.5. muuuy. Boum.

It is hereby certified-'trant error apbears vin the printed specification fof the`above numbered p'aterkxt requiring` correctionas follow-"fase 5, first' columx, line li-5, forfrestrietive" read --resistive--g'page 5,first col- I umn, line 52, claim iLrralined" read -lined--g and thmtthefauid4 Letters Patent y:should be read with v.this correction therein that the same may co nfom to the record of the case in the Patent Offic'e. I l u' signed and sealed this inch day of september, A. D. 19M.,

` f Henry Arsdale,

(Seal) 'Acting Comieioner of' Patents. 

